This invention relates to a process for obtaining desired materials from rocks or subterranean strata, the process using electrochemical treatment of some of the liquid streams.
It is known to remove a desired material (such as a metal) from rock in situ by means of a leachant or leaching liquid. The leachant is caused to flow through the rock, for example between spaced-apart boreholes, so the desired material dissolves in it; the leachate is then treated to extract the desired material. This process may be referred to as solution mining. For example as described in U.S. Pat. No. 5,523,066, lead may be removed from ore using a solution of acetic acid and hydrogen peroxide as the leachant, as lead acetate is soluble. This generates a solution containing various salts including lead acetate, and it is difficult to obtain the lead in a useful form without at the same time creating significant quantities of waste products.
According to the present invention there is provided a method of solution mining in which an ore is treated with a solution of acetic acid and hydrogen peroxide so as to form a leachate containing lead ions and the lead is stripped by solvent extraction from the leachate to form an aqueous solution containing lead ions, wherein the aqueous solution containing lead ions is treated electrochemically in the anodic compartment of a separated electrochemical cell to form a precipitate of lead oxide.
The leachate may also contain zinc ions. In this case the zinc is stripped by solvent extraction from the leachate to form an aqueous solution containing zinc ions, and the aqueous solution containing zinc ions is preferably then treated electrochemically in the cathodic compartment of a separated electrochemical cell to form a precipitate of zinc hydroxide.
The leachate may also contain manganese ions. After stripping the manganese by solvent extraction from the leachate to form an aqueous solution containing manganese ions, the aqueous solution containing manganese ions is treated electrochemically in the anodic compartment of a separated electrochemical cell to form a precipitate of manganese dioxide.
In each case the aqueous solution containing metal ions, after passing through a respective compartment of the respective separated electrochemical cell, is preferably recirculated back to the same compartment via a separation means to remove the precipitate. The separation means may be a filter, or a settling vessel, or a cyclone or hydrocyclone.
In the case of the electrochemical cell to treat the aqueous solution of lead ions, and that used to treat the solution of manganese ions, the cathode compartment is preferably used to generate hydrogen peroxide (for use in making the leachant). This may be achieved directly, using a gas diffusion electrode as the cathode. Alternatively it may be achieved indirectly by generating hydrogen at the cathode which can be reacted with anthraquinone to form hydrogen peroxide; or using a water-soluble anthraquinone derivative that can be reduced at the cathode and subsequently reacted with air to form hydrogen peroxide.
In a second aspect, the present invention provides a method of solution mining in which an ore is treated with a leachant containing hydrogen peroxide, so as to form a leachate containing metal ions, and the metal ions are stripped from the leachate by solvent extraction to form an aqueous solution containing the metal ions, wherein the aqueous solution is treated electrochemically in an anodic compartment of a separated electrochemical cell, and the cathodic compartment of the cell is used directly or indirectly to generate hydrogen peroxide.
Typically, after removal of lead, zinc and manganese from the leachate, the principal cation remaining is calcium. The calcium ions can be removed, at least partially, by adding carbon dioxide to the solution, and then treating it electrochemically in the cathodic compartment of a separated electrochemical cell so as to precipitate calcium carbonate, which is then removed using a separation means, such as a hydrocyclone. If the leachate is treated to remove substantially all the metal ions, it may be regenerated as leachant.